1. Field of the Invention
The present invention relates to coupling communication signals to electrical power distribution systems.
2. Background of the Related Art
Radio frequency (rf) modulated data signals can be coupled to and communicated over medium and low voltage power distribution networks. Use of inductive couplers for this purpose is described in U.S. Pat. No. 6,452,482, entitled “Inductive Coupling of a Data Signal to a Power Transmission Cable”, and U.S. patent application Ser. No. 10/082,063, filed Feb. 25, 2002, entitled, “Coupling Broadband Modems to Power Lines”, both of which are assigned to the assignee of the present application, and the contents of which are incorporated herein by reference.
Power distribution networks are occasionally subject to significant transients in voltage and current. For example, a strong current pulse of fast rise time is created when a power line device such as a distribution transformer short circuits, or when power lines fall and touch each other. Similarly, a lightning strike to a nearby point on the power line generates a traveling wave on the power line. A standard method of simulating a lightning strike is the Basic Impulse Loading (BIL) pulse, used for testing power line devices that would be connected to power lines, and it has a rise time of 1.2 microseconds, with a much longer fall time. The amplitude of such test pulses can vary between 90 and 200 kV peak.
A power line inductive coupler is basically a transformer whose primary is connected to the power line and whose secondary is connected to a communications apparatus such as a modem. The primary winding has one or just a few turns and presents a very low impedance at a power frequency. However, the coupler is capable of coupling the high frequency energy represented by the fast onset of a lightning pulse or other transient, and substantial voltage would be induced in the coupler secondary circuit.
Coupler flashover of medium voltage from a primary power wire to ground occurs when the wire's voltage exceeds the insulation capability of the coupler, whether during normal operation or during transient voltage pulses originating in lightning strikes or switching transients. Flashover can occur on the outer surface of the coupler or internally between parts of the coupler. Flashover may be considered a very rare event for suitably insulated devices attached to a medium voltage power line. For example, current and potential transformers commonly used by utilities often do not carry special protective circuitry. But in the case of a data coupler, which is intended to be used ubiquitously for a large customer base, it is considered prudent to protect against rare events, to prevent injury or damage.
In addition, since the modem is connected to lines leading to customer equipment, the modem is grounded. Therefore, the distribution power voltage must be insulated from the modem. If the inductive coupler's secondary were insulated from ground, then the voltage difference between the power line and ground would be divided across (a) the coupler's primary to secondary insulation and (b) the insulation of other devices in the chain of devices leading to the modem. The voltage drops would be proportional to the impedances across each insulation interface, and thus inversely proportional to the stray capacitance across each such interface.
When dealing with medium voltage ac power lines, with voltages in excess of 2,000 volts rms relative to neutral or ground, this capacitive voltage division would be difficult to make deterministic, as the coupler capacitance would depend on the position and diameter of the power line within the coupler. Therefore, any other insulating devices would need to be capable of insulating the full power line voltage, and thus be large and expensive.